2.28 Measurement Supervision
169
7UM61 Manual
C53000-G1176-C127-3
Thereby I
max
is the largest of the three phase currents I
min
the smallest. The symmetry
factor BAL. FACTOR I represents the allowable asymmetry of the phase currents
while the limit value BALANCE I LIMIT is the lower limit of the operating range of
this monitoring (see the following figure). Both settings are adjustable. The dropout
ratio is about 95 %.
This malfunction is signaled as “Fail I balance”.
Figure 2-64 Current symmetry monitoring
Voltage Symmetry From the phase-to-ground voltages, the rectified average value is formed as a check
for symmetry of absolute values. The smallest phase voltage is compared to the larg-
est. Asymmetry is recognised if
| U
min
| / | U
max
| < BAL. FACTOR U as long as | U
max
| > BALANCE U-LIMIT
Thereby U
max
is the highest of the three voltages and U
min
the smallest. The symmetry
factor BAL. FACTOR U is the measure for the asymmetry of the conductor voltages;
the limit BALANCE U-LIMIT is the lower limit of the operating range of this monitoring
(see following figure). Both settings are adjustable. The dropout ratio is about 95 %.
This malfunction is reported as “Fail U balance”.
If the 90% stator earth fault protection functions are active, a zero voltage results on
voltage asymmetry. If this cause protection pickup, monitoring is relegated to the back-
ground and issues no indication.
Figure 2-65 Voltage symmetry monitoring
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